We and others have shown that to initiate and maintain the growth and differentiation of primitive progenitor cells, multiple cytokine stimulation (synergy) is required. More recently, we showed that such cooperation also occurs between negative regulators of cell growth, and that the ability of primitive progenitors to proliferate depends on the balance of positive and negative signals the cell receives. Transforming growth factor beta (TGF?) directly and reversibly inhibits hematopoietic stem cells with marrow repopulating ability (LT-HSC). Also, short-term incubation with antibody and antisense to TGF? stimulates the self-renewal potential of these stem cells. TGF? has inhibitory effects on the cell surface expression of many cytokine receptors that directly correlates with its effect on cell growth. For example, stem cell factor receptor (c-kit) expression is downregulated by TGF?, in part by affecting c-kit mRNA stability. These results indicated that c-kit expression couldbe negatively regulated on LT-HSC. Indeed, we were able to characterize a novel LT-HSC lacking c-kit expression and that in bone marrow cell development, this cell matures into a c-kit+ LT-HSC. Also, TGF? prevents S phase cell-cycle progression through an intracellular mechanism involving regulation of transcription factors and cell-cycle regulatory proteins. In vivo results demonstrated that TGF? can protect mice from both the lethal hematopoietic toxicity of 5-FU, as well as the nonhematopoiesis toxicity of DXR. These findings show that a negative regulator of hematopoiesis can be successfully used systemically to mediate chemoprotection in vivo. Previous results from many labs also indicated that TGF? treatment of donor cells before bone marrow transplantation (BMT) could have a beneficial effect by blocking the immune reactivity. We were able to show suppression of graft vs host disease (GVHD) after allogeneic BMT through a TGF? mediated mechanism. Treatment of donor CD4 T-cells with TGF? and IL-10 made the donor T-cells hyporesponsive and less able to promote GVHD. In many instances, growth inhibition following terminal differentiation or anti-cancer drug treatment results in apoptosis (programmed cell death).